1. Field
The present disclosure relates generally to intelligent electronic devices (IEDs) for electrical power systems, and more particularly, to an intelligent electronic device having circuitry for an input structure of an analog-to-digital converter (ADC) that reduces noise of a signal from a sensor in the device, resulting in a highly accurate power measurement.
2. Description of the Related Art
Electric utility companies (“utilities”) track electric usage by customers by using power meters, also known as intelligent electronic device (IEDs). These meters track the amount of power consumed at a particular location. These locations range from power substations, to commercial businesses, to residential homes. The electric utility companies use information obtained from the power meter to charge its customers for their power consumption, i.e., revenue metering.
A popular type of power meter is the socket-type power meter, i.e., S-base or Type S meter. As its name implies, the meter itself plugs into a socket for easy installation, removal and replacement. Other meter installations include panel mounted, switchboard mounted, and circuit breaker mounted. Additional meter forms include switchboard drawout forms, substation panel metering forms, and A-base front wired forms. Typically, the power meter connects between utility power lines supplying electricity and a usage point, namely, a residence or commercial place of business.
A power meter may also be placed at a point within the utility's power grid to monitor power flowing through that point for distribution, power loss, or capacity monitoring, e.g., a substation. These power and energy meters are installed in substations to provide a visual display of real-time data and to alarm when problems occur. These problems include limit alarms, breaker control, outages and many other types of events. Conventionally, the visual display includes numerical information and/or an alarm indication, e.g., a LED, on the face of the meter.
To determine power consumed in a system, a power meter measures or senses current and voltage of at least one phase of the power distribution system. Voltage transformers, or potential transformers, in the power meter are used to sense or measure the voltage. The sensed voltage signal is transmitted to an analog-to-digital converter (ADC) in the power meter for conversion from analog to digital data, which is stored or processed by the power meter.
Common-mode noise, which can be caused by noise from power lines, power supply ripple, electromagnetic fields, radio frequencies, or high-frequency switching noise, for example, may be present in the voltage transmitted to the voltage input ADC. This noise causes loss of accuracy in meter readings, which can negatively impact the meter's ability to be used for revenue and billing purposes. Various ways of reducing this noise, thereby improving meter accuracy, have been implemented in the art. These solutions have involved changes to the voltage input ADC in the attempt to improve accuracy by reducing common-mode noise.
The structure of the voltage input ADC can be one of three types: single-ended, pseudo-differential, and fully-differential. Additionally, some ADCs known in the art can be configured as either single-ended or pseudo-differential voltage inputs (such as model MAX186 and MAX147 from Maxim Integrated Products, Inc. of Sunnyvale, Calif.); or single-ended or fully-differential voltage inputs (such as model MAX1298 and MAX1286 from Maxim Integrated Products, Inc. of Sunnyvale, Calif.).
Each of these types of voltage input ADCs has benefits as well as drawbacks. With the single-ended voltage input ADC, all voltage signals are referenced to a common ground at the ADC, with each voltage channel using a single input pin. An example of a single-ended voltage input ADC is depicted in FIG. 1A. If the signal source and the ADC are in close proximity, this structure works well. However, DC offset and noise in the signal path decrease the dynamic range of the voltage input signal. The single-ended voltage input ADC has the advantage of being the most inexpensive of the ADC types, but it may not yield accurate enough results for the billing or revenue functions of a power meter.
The pseudo-differential voltage input ADC separates signal ground from the ADC ground, allowing DC common-mode voltages to be cancelled, but this structure does not compensate for AC dynamic common-mode noise. An example of a pseudo-differential voltage input ADC is depicted in FIG. 1B. Even though the pseudo-differential voltage input ADC produces data with greater accuracy than the single-ended structure, it does not filter out all common-mode noise.
The fully-differential voltage input ADC offers maximum noise rejection of the three ADC structures mentioned. An example of a fully-differential voltage input ADC is depicted in FIG. 1C. Like the pseudo-differential ADC, the fully-differential ADC separates signal ground from the ADC ground, rejecting DC common-mode voltages. This structure ADC also offers dynamic common-mode noise rejection, by measuring the difference in voltage between the positive and negative terminals of the voltage sensor. However, the fully-differential input voltage ADC is both more complex and more expensive than the other two structures.
Therefore, there is an increasing demand in the electrical metering industry for devices, such as electrical power meters, analyzers, etc., to be able to accurately measure AC current and its related parameters, e.g., power, energy, power factor, etc., with higher dynamic range and higher precision without incurring significant additional complexity and cost of a implementing fully differential input voltage paths to the ADC.